A Prospective: Quantitative Scanning Tunneling Spectroscopy of Semiconductor Surfaces

Analysis methods that enable quantitative energies of states to be obtained from vacuum tunneling spectra of semiconductors are discussed. The analysis deals with the problem of tip-induced band bending in the semiconductor, which distorts the voltage-scale of the spectra so that it does not correspond directly to energy values. Three-dimensional electrostatic modeling is used to solve the electrostatics of the tip-vacuum-semiconductor system, and an approximate (semiclassical in the radial direction) solution for the wavefunctions is used to obtain the tunnel current. Various applications of the method to semiconductor surfaces and other material systems are discussed, and possible extensions of the method are considered.</p

Comparison of electronic and mechanical contrast in scanning tunneling microscopy images of semiconductor heterojunctions

The use of cross-sectional scanning tunneling microscopy (STM) to study strain in semiconductor heterostructures is discussed. In particular, intermixing between constituent heterostructure layers leads to internal strains in the heterostructure, and these strained regions are evident by displacement of the cleavage surface formed in the STM study. A theoretical analysis is made of the magnitude of electronic compared to mechanical contributions to the contrast of STM images, from which it is found that the former are relatively small, on the order of 0.1Å, for typical InxGa1−xAsyP1−y heterostructures imaged with sufficiently large, positive sample bias.</p

Electrostatic Potential for a Hyperbolic Probe Tip near a Semiconductor

The electrostatic potential resulting from a metallic probe tip near a semiconductor is examined. A solution is formulated assuming circular symmetry and using prolate spheroidal coordinates in the vacuum and Cartesian coordinates in the semiconductor. The result is most directly applied to the case of a hyperbolic probe tip, but other shapes (for example, a small hemispherical protrusion on the tip apex) can also be handled. Numerical results are given for representative cases that might be encountered in scanning probe microscopy

Quantitative Determination of Nanoscale Electronic Properties of Semiconductor Surfaces by Scanning Tunnelling Spectroscopy

Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evaluation of nanoscale electronic properties of the surface. Band offsets associated with quantum wells or quantum dots can thus be evaluated, as can be electronic properties associated with particular point defects within the material. An overview of the methods employed for the analysis is given, emphasizing the critical requirements of both the experiment and theory that must be fulfilled for a realistic determination of electronic properties.</p

Tunneling spectroscopy of graphene and related reconstructions on SiC(0001)

The 5×5,6√3×6√3−R30° and graphene-covered 6√3×6√3−R30° reconstructions of the SiC(0001) surface are studied by scanning tunneling microscopy and spectroscopy. For the 5×5 structure a rich spectrum of surface states is obtained, with one state, in particular, found to be localized on top of structural protrusions (adatoms) observed on the surface. Similar spectra are observed on the bare 6√3×6√3−R30° reconstruction, and in both cases the spectra display nearly zero conductivity at the Fermi level. When graphene covers the 6√3×6√3−R30° surface the conductivity at the Fermi level shows a marked increase, and additionally the various surface state peaks seen in the spectrum shift in energy and fall in intensity. The influence of the overlying graphene on the electronic properties of the interface is discussed, as are possible models for the interface structure

Effects of hydrogen during molecular beam epitaxy of GaN

We study the effect of introducing hydrogen gas through the RF plasma source during plasma-assisted molecular beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present. But, the critical Ga flux for this transition is increased by the presence of H, and for sufficiently high H pressure a new 2 × 2 surface structure that is believed to be H-terminated is observed. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching on the surface, from which we argue that H selectively bonds to surface step and/or kink sites. </p

Low-energy Electron Reflectivity from Graphene: First-Principles Computations and Approximate Models

<p>A computational method is developed whereby the reflectivity of low-energy electrons from a surface can be obtained from a first-principles solution of the electronic structure of the system. The method is applied to multilayer graphene. Two bands of reflectivity minima are found, one at 0 – 8 eV and the other at 14 – 22 eV above the vacuum level. For a free-standing slab with n layers of graphene, each band contains n 1 zeroes in the reflectivity. Two additional imagepotential type states form at the ends of the graphene slab, with energies just below the vacuum level, hence producing a total of 2n states. A tight-binding model is developed, with basis functions localized in the spaces between the graphene planes (and at the ends of the slab). The spectrum of states produced by the tight-binding model is found to be in good agreement with the zeros of reflectivity (i.e. transmission resonances) of the first-principles results.</p

Low energy electron microscopy of indium on Si(0 0 1) surfaces

Low energy electron microscopy is used to study the behavior of thin indium films on Si(0 0 1) surfaces from 100 °C up to 700 °C. For temperatures below 150 °C we see inversions in the LEEM dark-field image and LEED 1/2-order spot intensities as indium coverage increases from 0 to 2 ML. For temperatures between 150 and 600 °C we find the formation of a disordered and an ordered (4 × 3) indium phase on the surface. For temperatures above 500 °C we observe significant rearrangement of the Si(0 0 1) surface due to the presence of indium and etching of the Si(0 0 1) surface by indium at temperatures greater than 650 °C.</p

Influence of surface states on tunneling spectra of n-type GaAs(110) surfaces

We show that surface states within the conduction band of n-type GaAs(110) surfaces play an important role in reducing the tunneling current out of an accumulation layer that forms due to an applied potential from a nearby probe tip. Numerical computation of the tunneling current combined with an electrostatic potential computation of the tip-induced band bending (TIBB) reveals that occupation of the surface states limits the TIBB, thus leading to the limitation of the accumulation. As a result, the tunneling current out of the accumulation layer is strongly suppressed, which is in quantitative agreement with the experiment.</p

Theory of resonant tunneling in bilayer-graphene/hexagonal-boron-nitride heterostructures

A theory is developed for calculating vertical tunneling current between two sheets of bilayer grapheneseparated by a thin, insulating layer of hexagonal boron nitride, neglecting many-body effects. Results are presented using physical parameters that enable comparison of the theory with recently reported experimental results. Observed resonant tunneling and negative differential resistance in the current–voltage characteristics are explained in terms of the electrostatically-induced band gap, gate voltage modulation, density of states near the band edge, and resonances with the upper sub-band. These observations are compared to ones from similar heterostructures formed with monolayer graphene.</p